Holography is a technique that reproduces 3D images of objects. Holograms are images obtained by encoding a surface profile of a light field (amplitude and phase) rather than light intensity. A reference light beam is used to write the hologram profile, and then to reproduce the original image when applied to the profile.
Computer generated holograms have been introduced which eliminate the need to write holograms using interference of two light sources. In some applications, computed holograms are patterned on films using lithography techniques.
With the advances of metasurfaces composed of plasmonic nano-antennas, ultrathin subwavelength holograms have been obtained by utilizing the ability of plasmonic nano-antennas to introduce an abrupt change to the phase of the incident beam.
Monochromatic ultrathin metasurface holograms have also been developed where antennas dimensions were designed to tailor the induced phase-shift, therefore enabling the development of monochromatic phase holograms. Most of these holograms have binary amplitude modulation (corresponding to existence or absence of nano-antennas).
Monochromatic holograms have a maximum storage capacity due to the diffraction limitation imposing a minimum distance of λ/2 between antennas to be distinguishable in the far-field.
Binary color holograms have been produced to increase the maximum capacity of the encoded hologram beyond the diffraction limit. In this structure, antennas dimensions are tailored to encode wavelength (color), instead of phase. However, such binary color holograms only enable binary encoding of data (existence or absence antennas), and do not encode phase information. Therefore, improvements are needed in the field.